The invention relates to an injector for applying fluids, particularly contrast media for x-ray or nuclear-spin tomography, with a tube, as well as a roller pump, at least partially surrounded by the tube in peripheral direction, for the transport of the fluid from a storage vessel to a cannula.
In such injectors known from practice roller pumps are used for the transport of the fluid in the tube from the storage vessel to a patient, without direct contact, thereby insuring sterility. However there have been problems because the roller pump does not work free of slippage and the output depends on the counterpressure so that, particularly when unsuitable cannulas with a diameter which is too small are used, the amount of fluid required for the examination is not injected to the patient, although the roller pump works with the selected predetermined drive output, respectively rotations per minute.
It is therefore the object of the invention to develop an injector of the kind mentioned in the introduction, which during the application of fluids insures at all times that the patient receives the amount of fluid required for the examination.
This task is achieved in an injector of the aforementioned kind by providing a pressure chamber connected with the tube interior through an opening in the tube wall, having a component which can be adjusted under the effect of the fluid pressure and which acts upon a pressure sensor.
This invention offers the advantage that the flow rate of the fluid does not have to be measured directly, since it has been found that below a certain pressure threshold value, there is a sufficiently linear interrelationship between the output of the roller pump and its rotary speed, so that it is sufficient to monitor the pressure ratio in order to insure that the pressure value threshold is not surpassed. It has also to be noted that the sterility of the fluid is furthermore insured while the pressure is measured, because there is no direct contact of the fluid with the pressure sensor, since the pressure ratio is being transmitted to the latter by the adjustable component.
In a roller pump one roller is in contact with the elastically deformable tube and deforms the same, in order to transport the downstream located fluid column further upstream in the rotation direction of the pump. However the elasticity of the tube influences the precision of the pressure measurement, so that, according to a further development of the invention, it is provided that between two segments of the tube a rigid pipe with an opening be inserted, to which the pressure chamber is fastened.
An embodiment distinguished by the simplicity of its construction is characterized in that the adjustable component consists of a liquid-proof elastic membrane. This elastic membrane in contact with the pressure sensor stretches under increasing pressure, respectively shrinks back under falling pressure, a fact communicated to the pressure sensor, which based on the changes of the membrane can determine the pressure ratio.
If sufficient measuring accuracy is required over a longer measuring interval, it is advantageous to design the adjustable component as a piston, which acts upon the pressure sensor with a prop tightened against the bottom plate of the pressure chamber. In this embodiment there is no limitation of the adjustment range of the prop, respectively piston, stipulated in principle, so that measurements can be performed over a longer pressure interval.
The same advantage exists also when the adjustable component is a piston tightened against the pressure chamber wall, which acts directly upon the pressure sensor, whereby in this embodiment the simplicity of the construction is further enhanced.
Since the injector is intended for use in the medical field, there are high requirements for its reliability. Since the detection of the pressure ratio with the pressure-measuring system is to be conceived as a safety feature, it is advantageous when the pressure chamber and the associated pressure sensor are doubled, the safety devices being this way laid out to be redundant.
In principle it is sufficient when the pressure chamber is connected through a single opening in the tube wall with the tube interior, since according to the principle of communicating tubes, pressure changes inside the tube would be transmitted also to the pressure chamber, in that for instance fluid would flow into the pressure chamber when the pressure rises and when it falls the flow direction would reverse itself. However if only one opening is available, it can take a relatively long time until the ratios have again stabilized after a pressure change, so that the time resolution would be relatively minimal. In order to improve the latter, it is provided that several openings of the tube wall are assigned to the pressure chamber. In the case of several openings the fluid not only flows into the pressure chamber and remains there stationarily at constant pressure ratios, but a flow passes through the pressure chamber, where through one opening the fluid flows into the pressure chamber and exits through another opening, so that the pressure-measuring system can follow the pressure changes more promptly. It is also advantageous that the fluid in the pressure chamber is constantly exchanged, so that when a fluid is changed, for instance from one contrast medium to another or to a rinsing solution, there are no residues of the previous fluid in the tube.
It has also proven to be advantageous when the pressure chamber is arranged on the cannula-side end of the tube, since concurrently with the pressure measurement also a detection of the seal tightness of the upstream portion of the tube can take place, therefore suitably the pressure chamber is arranged as far upstream as possible.
In order to test the function capability and the integrity of the pressure-measuring system, it is favorable when the tube with the pressure chamber and the pressure sensor passes through a container, wherein a wetness sensor is arranged as a leak detector.
According to a particularly advantageous embodiment it is further provided that the pressure sensor consists of an electric pressure receiver, since this way the pressure sensor not only indicates the pressure value at a particular moment, but also creates the possibility of a further evaluation of the signal of the electric pressure receiver, for which purpose an evaluation unit controlling the rotary pump speed is provided for the electric signals sent by the electric pressure receiver.